{"id":404,"date":"2012-10-20T16:13:45","date_gmt":"2012-10-20T21:13:45","guid":{"rendered":"http:\/\/tonykordyban.com\/?page_id=404"},"modified":"2014-04-13T20:28:15","modified_gmt":"2014-04-14T01:28:15","slug":"everything-you-know-is-wrong-october-2002","status":"publish","type":"page","link":"http:\/\/tonykordyban.com\/?page_id=404","title":{"rendered":"Everything You Know Is Wrong October 2002"},"content":{"rendered":"

Answers to those Doggone Thermal Design Questions<\/strong><\/p>\n

By Tony Kordyban<\/strong><\/p>\n

Copyright by Tony Kordyban 2002<\/p>\n

Dear Tony,<\/em><\/p>\n

Long time reader, first time writer. I am seeking design information to size a chimney for use in an a electronics packaging application. Can you point me to any existing work, or even better, provide me with the equations? CFD is not an option at this time.\u00a0<\/em><\/p>\n

Bert from Mary Poppins<\/em><\/p>\n

 <\/p>\n

Dear Bert,<\/p>\n

If you mention the phrase “natural convection” at the cafeteria lunch table, somebody is bound to respond with, “ah, yes, the chimney effect.”\u00a0 I find it curious that everybody knows about “the chimney effect” except me.\u00a0 None of my college heat transfer courses mentioned such a thing.\u00a0 (There was a course in power plant design, and it did mention that you want to have very tall chimneys on your coal-fired boilers so that the pollution ends up in another legal jurisdiction, but that is a totally different “chimney effect”.)<\/p>\n

\"\"<\/a>

Figure 1. A very, un-chimney-like electronic chassis. There is very little air flow through the horizontally oriented vents. The buoyancy of the hot air mostly generates small recirculating currents inside the box.<\/p><\/div>\n

Heat transfer specialists have a special jargon for the “chimney effect”:\u00a0 gravity.\u00a0 In natural convection, a hot object heats up the surrounding air.\u00a0 The hot air is less dense than the surrounding cold air.\u00a0 Then, as everybody knows, “hot air rises.”\u00a0\u00a0 Actually, it is the other way around.\u00a0 Cold air falls.\u00a0 Gravity pulls more on cold air, because it is denser.\u00a0 The heavier, colder air is pulled to the bottom of the pile of air, and that pushes the hot air upward out of the way.\u00a0 The “buoyancy effect” is really just gravity sorting out air, pulling the heaviest air down close to the earth and letting the lightest air pop up to the top of the stack.<\/p>\n

So because of gravity, cold air moves downward and hot air moves upward.\u00a0 What does that have to do with a chimney?\u00a0 Let’s say you have an electronics chassis like the one in Figure 1.\u00a0 There are vents at opposite ends of the chassis, but no fan.\u00a0 The electronics inside generate heat, so soon there is a both hot and cold air inside the<\/p>\n

\"\"<\/a>

Figure 2. When you line up the heat source and the vents with the direction that the air wants to flow anyhow, you get what is called "the chimney effect."<\/p><\/div>\n

chassis.\u00a0 But because there is no height difference between the two vents, there is no tendency for the buoyant hot air, which want to move up, to leave the box at all.\u00a0 It stagnates against the inside of the roof of the chassis.
\n<\/em><\/p>\n

Figure 2 shows the same chassis turned 90 degrees, so that one vent is lower than the other. (Even a small angle would be enough to get the flow started.)\u00a0\u00a0 Now the rising hot air can easily flow right out of the upper vent, and cooler ambient air can replace it by flowing into the lower vent.\u00a0 A natural flow is established through the box, and this increases the rate of heat transfer and lowers the temperature of the electronics.\u00a0 Natural convection works a lot better for vertically vented boxes such as in Figure 2 than for horizontal boxes such as in Figure 1.\u00a0 A vertical box with vertical flow looks more like a chimney, so I suppose that is where the phrase “chimney effect” comes from.<\/p>\n

Gravity is the reason that the vertical direction works and horizontal doesn’t.\u00a0 For natural convection, the inlet and exit vents need to have vertical separation, and usually, the bigger the separation, the better.<\/p>\n

But, Bert, you already knew all that.\u00a0 I just wanted to make sure that people didn’t think there was something magic about the shape of a chimney that made it generate air flow all by itself.\u00a0 The heat creates the density change, and gravity provides the force to make the air flow.\u00a0 The “chimney” is just an enclosure that doesn’t interfere with the direction the air wants to flow under the force of gravity.\u00a0 Now for those equations you’ve been waiting for.<\/p>\n

I am taking the equations and explanation here directly from Gordon Ellison’s book Thermal Computations for Electronic Equipment.\u00a0 If you have a copy, you’d be better off reading it there in Chapter 6.\u00a0 But because it is out of print, I’m going to give you the equations right here.<\/p>\n

\"\"<\/a>

Figure 3. The balancing point.<\/p><\/div>\n

Here is the method for finding the flow rate through a chassis in natural convection.\u00a0 It is similar to finding the flow rate through a chassis with a fan.\u00a0 The fan is a driving force that produces different flow rates at different pressures.\u00a0 Against higher pressure, there is less flow.\u00a0 This characteristic of a fan can be shown as a performance curve.\u00a0 Likewise, a curve can be found for the flow resistance of the chassis.\u00a0 The higher the pressure applied, the greater the flow.\u00a0 The flow rate and pressure drop (the operating point) for the combination of the chassis and the fan is where the fan curve and the system resistance curves cross.\u00a0 That is where the flow resistance balances out the driving force of the fan.
\n<\/em><\/em><\/p>\n

\"\"<\/a>

Figure 4. Circuit board in a chimney.<\/p><\/div>\n

I’m going to assume you know how to find the system resistance curve of your chassis.\u00a0 If not, search hard and get a copy of Ellison’s book.\u00a0 It’s all in there.\u00a0 But for natural convection, you don’t have a fan curve.\u00a0 What do you use for the driving force?<\/p>\n

A fan curve usually (in the US) gives the driving force in terms of pressure (inches of water) as a function of volumetric flow rate (cubic feet per minute or CFM).\u00a0 Ellison gives a pair of equations that give the same kind of curve for natural convection.\u00a0 It is not an empirical relation, but is derived from first principles.\u00a0 I’m not going to duplicate his derivation.\u00a0 Please find the book in a library if you want to see how he derives this.\u00a0 It is not hard to follow. <\/em><\/p>\n

The following equations assume the geometry in this figure.\u00a0 The chassis is oriented vertically so that air flows straight through from bottom to top.\u00a0 The heat source (in our case, a circuit board) gives off heat uniformly so that the air temperature rises linearly as it passes over the board.\u00a0 Note that the vertical dimension, d, is the height over which the temperature increases, not the height of the chassis.<\/p>\n

The increase in air temperature causes a change in air density, which generates a driving force for air flow.\u00a0 This force can be expressed as a pressure difference.\u00a0 The Ellison equations are:<\/p>\n

\"\"<\/a>\"\"<\/a><\/p>\n\n\n\n\n\n\n\n\n\n
<\/td>\n\n

where the terms are defined as:<\/p>\n<\/td>\n<\/tr>\n

\n

\"\"<\/a><\/p>\n<\/td>\n

\n

is the pressure difference in units of inches of water (in. H2O)<\/p>\n<\/td>\n<\/tr>\n

\n

d<\/p>\n<\/td>\n

\n

is the vertical length of the heat source, in inches<\/p>\n<\/td>\n<\/tr>\n

\n

\"\"<\/a><\/p>\n<\/td>\n

\n

is the temperature rise of the air over the length of the heat source, in degrees C.\u00a0 It is not the temperature rise of the heat source.\u00a0 This can also be thought of as the exit temperature – inlet temperature<\/p>\n<\/td>\n<\/tr>\n

\n

T0<\/sub><\/p>\n<\/td>\n

\n

the inlet air temperature, in \u00b0C<\/p>\n<\/td>\n<\/tr>\n

\n

G<\/p>\n<\/td>\n

\n

the volumetric air flow rate, in CFM (cubic feet per minute)<\/p>\n<\/td>\n<\/tr>\n

\n

Q<\/p>\n<\/td>\n

\n

is the heat flow rate of the heat source, in watts<\/p>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

You don’t really need two equations.\u00a0 It’s just easier to write them out this way, plus you can see how the pressure is generated by the temperature rise.\u00a0 You can substitute the second equation into the first and get a formula for<\/p>\n

\"\"<\/a>

Figure 5. Pressure of natural convection flow.<\/p><\/div>\n

the pressure that only depends on four things:\u00a0 the vertical length of the heat source (d), the flow rate, the ambient temperature, and the power.\u00a0 You can use a simple spreadsheet to generate some pressure\/flow rate curves for your intended design, that might look like this:\u00a0 (for a heat source 12 inches tall)<\/p>\n

Notice that we got all this info about the driving force of natural convection without saying anything about the shape of the chimney, except for the vertical length of the heat source.\u00a0 Doesn’t the overall performance depend on things like the size of the inlet and exit vents?\u00a0 Or the cross sectional area of the chassis?<\/p>\n

Yes.\u00a0 But those things determine the system resistance to flow.\u00a0 The geometry of your vents and your box and your PCB determine what the system resistance curve looks like.\u00a0 Then you plot it on this same graph, and where they cross is the approximate operating point.<\/p>\n

Simple, eh?\u00a0 Maybe a little too simple.\u00a0 Most electronic boxes are not straight ducts, and most circuit boards are not uniform heat sources.\u00a0 So please remember that this is just a rough cut at estimating how a chassis will behave in natural convection.\u00a0 But it should be a pretty good cut, considering you didn’t have to invest in any CFD.\u00a0 You could probably do it even if you can’t afford a spreadsheet program.\u00a0 And if you don’t have a calculator, I have a used slide rule I can let you have, cheap.<\/p>\n


\n<\/em><\/p>\n

\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014\u2014<\/p>\n

Isn\u2019t Everything He Knows Wrong, Too?<\/strong><\/p>\n

The straight dope on Tony Kordyban<\/strong><\/em><\/p>\n

Tony Kordyban has been an engineer in the field of electronics cooling for different telecom and power supply companies (who can keep track when they change names so frequently?) for the last twenty years.\u00a0 Maybe that doesn\u2019t make him an expert in heat transfer theory, but it has certainly gained him a lot of experience in the ways NOT to\u00a0cool electronics.\u00a0 He does have some book-learnin\u2019, with a BS in Mechanical Engineering from the University of Detroit (motto:Detroit\u2014 no place for wimps) and a Masters in Mechanical Engineering from Stanford (motto: shouldn\u2019t Nobels count more than Rose Bowls?)<\/p>\n

\"\"In those twenty years Tony has come to the conclusion that a lot of the common practices of electronics cooling are full of baloney.\u00a0 He has run into so much nonsense in the field that he has found it easier to just assume \u201ceverything you know is wrong\u201d (from the comedy album by Firesign Theatre), and to question everything against the basic principles of heat transfer theory.<\/p>\n

Tony has been collecting case studies of the wrong way to cool electronics, using them to educate the cooling masses, applying humor as the sugar to help the medicine go down.\u00a0 These have been published recently by the ASME Press in a book called, \u201cHot Air Rises and Heat Sinks:\u00a0 Everything You Know About Cooling Electronics Is Wrong.\u201d\u00a0 It is available direct from ASME Press at 1-800-843-2763 or at their web site at\u00a0http:\/\/www.asme.org\/pubs\/asmepress<\/a>,\u00a0\u00a0<\/em><\/strong>Order Number 800741.<\/p>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

Answers to those Doggone Thermal Design Questions By Tony Kordyban Copyright by Tony Kordyban 2002 Dear Tony, Long time reader, first time writer. I am seeking design information to size a chimney for use in an a electronics packaging application. Can you point me to any existing work, or even better, provide me with the […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-404","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/404","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=404"}],"version-history":[{"count":4,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/404\/revisions"}],"predecessor-version":[{"id":415,"href":"http:\/\/tonykordyban.com\/index.php?rest_route=\/wp\/v2\/pages\/404\/revisions\/415"}],"wp:attachment":[{"href":"http:\/\/tonykordyban.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=404"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}